(1.) Field of the Invention
This invention relates to a resistance welding method using a high-frequency current for heating edges of a workpiece to produce a weld, wherein points to be welded are irradiated with a laser beam while heating with the high-frequency electric current is continued, so that welding can be performed efficiently by relying on heating by means of the high-frequency electric current and the laser beam.
(2.) Description of the Prior Art
Welding metals is a process that has been used in many fields and there are different welding methods available. In producing tubes by welding, a high-frequency welding method is most popular.
High-frequency electric resistance welding (ERW)is known as a method of welding of high welding-speed and productivity for producing what is generally referred to as an (ERW) tube.
In the process of producing welded tubes by means of a high-frequency electric resistance welding method of the prior art, a steel strip is formed into a cylindrical shape by means of an array of forming rolls and the edges of the strip in cylindrical form are butted against each other by means of a pair of squeeze rolls, so that the edges are coverged to form a wedge shape having a vertex at the butted end.
A high-frequency voltage is applied at contacters located upstream of the squeeze rolls to supply a high-frequency current from one contact to another contact, so that a high-frequency current is allowed to flow along the edges of the wedge shape. The edges are heated by the high-frequency current until reaching a welding temperature, when pressure is applied by the squeeze rolls to form a weld.
The quality of the weld produced in this way is greatly influenced by the amount of the welding current. When the welding current is too small, the edges have a low heat input, resulting in a weld defect generally referred to as a cold weld. Conversely, when the welding current is too large and the edges have a high heat input, a weld defect generally referred to as penetrator may sometimes be produced. Insufficient heating of the edges is mainly responsible for the cold weld produced. The principal cause of the penetrator is a periodic change in the positions of the points to be welded with respect to the axis of the tube due to a large amount of molten metal produced by too high a heat input and removed from the weld by the electromagnetic force.
The problems referred to hereinabove which the method of the prior art encountered will be described more in detail. Generally, a high-frequency electric current used for producing electrically welded tubes is in the range of frequencies between 10 and 500 KHZ. The effect of heating increases with higher frequency, due to the synergisms of the "skin depth" and the "proximity effect" characteristic of a high-frequency current. This is why a high-frequency current is favored in producing electrically welded tubes.
In high-frequency electric resistance welding, edges of a workpiece are melted by being heated with a high-frequency current and at the same time an upsetting force of high magnitude is exerted on the joint by means of a pair of squeeze rolls. This process is considered to involve the mechanism that welding is achieved as the major portion of the molten metal is squeezed out of the weld to outside together with oxides produced by heating. The weld is deformed by the upset, and a metal flow rises in the heat-affected zone, as shown in FIG. 2.
A rise in metal flow results in a simultaneous rise in inclusions in the strip metal. This gives rise to the defect that the interior of the metal which is inferior in mechanical and chemical properties to the surface portion is exposed. Meanwhile, when no upsetting is applied, weld defects are produced. FIG. 3 shows the relation between the rising angle .theta. of metal flow and the toughness of the weld. The larger the rising angle .theta., the lower becomes the toughness. When the rising angle .theta. is small, toughness may show variations due to the defective weld defect, with the value of toughness dropping to an unordinarily low level. In FIG. 3, a hatched area represents the zone of toughness. Toughness varies in the hatched area. The metal flow rising angle has been considered to be acceptable when it was in the range between 50 and 70 degrees.
A high-frequency current is concentrated on the surface of the edges which are butted together, particularly in the corners. Thus, the metal is melted in greater amounts in the corners than in the central portion of the butted edges. The molten metal produced at the edges is removed from the edges to outside by the action of the electromagnetic pressure induced by the welding currents. FIG. 4 shows the directions of the electromagnetic pressure, and FIG. 5a shows the shape of the butted edges immediately before welding is performed. It will be seen that each edge is convex in surface, with its central portion being protuberant. Immediately after welding is performed, molten steel fills the gap between the edges. If the molten steel were allowed to solidify in this condition or without any upsetting force being applied on the weld, pores would be produced near the corners due to solidification shrinkage of the molten steel, rendering the weld defect. FIG. 5b shows this weld defect. If an upsetting force of high magnitude were applied on the weld, the weld would be deformed into a planar shape, and the layer of solidified metal would be in the form of a thin film and no cavities would be formed due to shrinkage, as shown in FIG. 5c.
In resistance welding using a high frequency current of the prior art, an intense upsetting force should be applied to avoid the weld defects as described hereinabove. However, an intense upsetting force has given rise to the problem that the rising angle .theta. of the flow metal becomes large and the weld becomes lower in toughness.
This phenomenon has been observed not only in producing electrically welded tubes of straight seam but also in performing electrical resistance welding of spiral tubes.
Another reason why it has been difficult to perform welding with a low upsetting force is that the edges of the workpiece are nonuniformly melted due to nonuniform distribution of a high frequency current. It has been found that if the edges could be uniformly melted it would be possible to perform welding with a low upsetting force. It has also been found that it is in about 20% of plate thickness from the corners that the electrical current is concentrated to cause excessive melting of the steel, and that the steel is melted almost uniformly in the center range between 1/4 and 3/4 of plate thickness.
Meanwhile, a welding method using a laser beam or electron beam is available for producing a sound weld with a minimum heat affected zone. Proposals have been made to use the welding method in which a beam of such radiant energy is applied to the vertex of a wedge constituting a point to be welded, as disclosed for example in Japanese Patent Application No. 107120/83.
The welding process described in this document will be outlined. Edges (wedge-shaped opposed faces to be welded) of a tubular member are heated uniformly to a welding temperature through the entire range of thickness by the Joule heat generated by a high-frequency current supplied through a contact and by a laser beam applied through a beam guide from a laser unit.
The laser beam reciprocatorily scans the opposed edges of the tubular member in a predetermined range of angles with the vertex of the wedge of a predetermined angle constituting a weld point being disposed in the center. The laser beam is projected against one of the opposed edges and reflected thereby to be projected against the other opposed edge, and this reflection of the laser beam is repeated until reaching the weld point. Stated differently, even if the laser beam is not directly applied to the weld point, the beam is reflected to converge until it is automatically concentrated upon the weld point.
When this method of combined welding was used, difficulties have been experienced in achieving a predetermined heating pattern due to variations in the position which is irradiated with a laser beam caused by variations in the point to be welded stemming from variations in the thickness and strength of the material of the tubular member. It has been found that this phenomenon significantly increases in incidence when the steel material has a large thickness. Also, the phenomenon has been found to occur when there are some problems with regard to the uniformity of steel material, setting of the position of the beam and the shape of the projected beam. The phenomenon may occur, even if the position of the beam is set correctly, due to mechanical variations in the position, particularly errors in the shape of the tubular member, variations in the position of the tubular member and variations in the position of the energy beam emitting unit.